The present invention generally relates to parallel processing of information and more particularly to a parallel optical information processing system suitable for character identification, visual image sensing and the like.
As an outcome of demand for a system which is capable of identifying images having predetermined patterns such as numeric or alphabetical characters at a high speed, a parallel optical information system has recently been developed. In this system, an object light forming an input image is projected on a hologram carrying a plurality of Fourier transform images recorded thereon by multiple exposure. When the foregoing object light is incident to such a hologram, an optical output is formed as a result of diffraction such that on of the Fourier images which approximates the incident image with a maximum correlation is reproduced with a maximum intensity. Using such a nature of the hologram, the system performs the identification of the images.
FIG. 1 shows a typical prior art parallel optical information processing system 1. Referring to the drawing, the system 1 comprises an information input subsystem which in turn comprises a laser diode 2 for producing a coherent optical beam along an optical axis, a collimator lens 3 provided on the optical axis for shaping the coherent optical beam from the laser diode 2 into a parallel optical beam, and a spatial modulator 4 also provided on the optical axis for modulating the parallel optical beam from the collimator lens 3. On the optical axis of the optical beam, there is further provided a half-mirror 5 with an angle of 45 degrees with respect to the optical axis, and beyond the half-mirror 5, there are provided a first Fourier transform lens 6 and a Fourier transform hologram 7 acting as a hologram memory. Further, beyond the hologram 7, there are provided a second Fourier transform lens 8 and a pinhole array mirror 9 along an optical path of a diffraction beam produced by the hologram 7. On an optical path of a reflection beam produced by the foregoing half-mirror 5, on the other hand, there is provided a detector 10 which may be a charge-coupled device (CCD) which in turn is connected to an image identification apparatus 13 and further to the foregoing spatial modulator 4 via an amplifier 11 and a threshold device 12. Also, the spatial modulator 4 is coupled to an image input device (not illustrated) having a photo-electric conversion function.
In the foregoing system, an input image to be processed (not illustrated) is first converted to an electrical image by the not-illustrated image input device and is written into the spatial modulator 4. When the parallel coherent optical beam is incident to the spatial modulator 4 from the laser diode 2 via the collimator lens 3, an object light is produced as a result of modulation of the beam by the spatial modulator 4. This object light is then subjected to the Fourier transformation in the first Fourier transform lens 6 and is projected to the Fourier transform hologram 7. The Fourier transform hologram 7 is recorded with a number of Fourier transform hologram images in an overlapped manner by multiple exposure with an angle of the reference light which is changed for each hologram image at the time of recording, and produces a first order diffraction beam which is then focused by the second Fourier transform lens 8 on the pinhole array mirror 9. The first order diffraction beam thus focused is then reflected and returned to the Fourier transform hologram 7 by passing through the lens 8 along a reversed optical path. Thereby, one of the Fourier transform images which has the maximum correlation to the incident image is reproduced with maximum intensity. This reproduced Fourier transform image is then detected by the CCD device 10 and is converted to an electrical signal. This electrical signal is then supplied to the apparatus 13 for image identification after amplification and suitable correction in the amplifier 11 and the threshold device 12. In a case when the identification in the apparatus 13 turned to be difficult, an output of the apparatus 13 carrying the reproduced image is supplied again to the spatial modulator 4 and the foregoing procedure is repeated.
In the prior art parallel optical information processing system as described, there is an advantage in that a complete image can be reproduced from an incomplete image. Further, such a system can be used as an associative memory.
Presently, such a parallel optical information processing system is required to perform various functions, and associated therewith, it is strongly desired that the Fourier transform hologram 7 used in the system is rewritable. Conventionally, the Fourier transform hologram 7 has been recorded with the hologram images as a result of interference between an object light carrying the Fourier transform images and a reference light while changing the angle of the reference light for each of the Fourier transform images. For such a purpose, a complex mechanism is needed and the rewriting of the hologram image has been difficult. Further, storing of many informations in one hologram is difficult as the recording of the Fourier transform image is made by the multiple exposure of the interference patterns. When the number of the overlapped Fourier transform holograms is increased, the entire hologram becomes dark and the separation of the individual diffraction beams becomes difficult. Because of this reason, the number of the Fourier transform images that can be held in one hologram has been limited to five or six in the maximum. Further, as a consequence that the prior art parallel optical processing system processes the images of bodies directly, further increase of the information to be processed by the system is difficult.